1. Field Of The Invention
This invention relates to an aqueous coating composition containing a film forming material based on a reaction product of carboxy functional polycondensates and .alpha.,.beta.- olefinically unsaturated monomers together with a polyurethane dispersion and optionally grinding resins and/or grinding auxiliaries.
2. Description Of The Prior Art
Coating compositions containing reactive polymer dispersions are described in DE-OS No. 3 128 025 and DE-OS No. 2 128 062. They give rise to weather-resistant coatings but show insufficient adhesion when used in multilayered lacquers.
Carboxy-functional self-curing polymers which additionally contain epoxy groups, for example prepared according to DE-OS No. 2 811 918, give rise to firmly adhering coatings when used in combination with melamine resins in metallic base coat formulations (according to DE-OS No. 3 301 729) but these coatings have undesirable properties, for example with regard to their storage in water. This applies in particular to the build-up of lacquers under so-called repair conditions in which additional layers of lacquer are applied to already previously lacquered surfaces at low curing temperatures. The polymers mentioned. however, do not have satisfactory flow properties even when thickeners are added.
When used in combination with reactive melamine resins in aqueous metallic basecoat coatings, mixtures of polymer dispersions, e.g. of the kind described in the above-mentioned DE-OS Nos. 3 128 025 and 3 128 062, with the above-mentioned self-curing polymers do not provide any improvement in the desired combination of properties of adherence to intermediate layers and stability in water. On the contrary, a combination of poor interlayer adherence and insufficient storage stability in water generally occurs. Moreover, this mixture cannot be sufficiently thickened with the usual thickeners based on polyacrylates. polyethylene oxide, polyvinyl alcohols, cellulose ethers and others.